1. Technical Field
The present invention relates to a solution searching system by quantum dots for searching a solution of an optimum solution search problem by use of a nanoscale circuit made of quantum dots.
2. Related Art
In recent years, a nanoscale arithmetic circuit capable of performing logical operation processing without a dominant diffraction limit of light is desired in order to meet requests for future large-capacity information processing. However, there is a problem that a quantum fluctuation occurs when the nanoscale circuit is to be realized in an electronic device, and there is a problem that minimization is restricted due to the diffraction limit of light when the nanoscale circuit is to be realized in an optical device.
There is therefore proposed an arithmetic circuit using nanoscale quantum dots making maximum use of the granularity of electrons by controlling single electrons. The quantum dot is a box which is formed to have a potential sufficient to give 3D quantum confinement to exciters by use of a semiconductor microfabrication technique. By use of the exciter confinement system, an energy level of a carrier in a quantum dot is discrete and a state density is sharpened in a delta function manner. A single-electron memory using light absorption in a sharpened state of the quantum dot or a single-electron transistor for powering ON/OFF single electrons for entrance/exit of quantum dots has been already studied, and a single-electron nanoscale operation is to be realized.
There is proposed an arithmetic circuit using quantum dots particularly in recent years (see JP 2006-023505 A, for example). For example, as illustrated in FIG. 12, an arithmetic circuit made of quantum dots is formed of a second quantum dot 213 at one output side formed on the surface of a substrate, and a plurality of first quantum dots 212 discretely formed around the second quantum dot 213. A light intensity of an output light from the second quantum dot 213 is changed depending on input lights supplied to the first quantum dots 212. The light intensity of the output light from the second quantum dot 213 is controlled by the amount of discharged exciters to a lower level in the second quantum dot 213, and the amount of discharged exciters depends on the amount of exciters transmitted via resonant levels from the quantum dots 212a and 212b. That is, when signal lights A and B are supplied to all the first quantum dots 212a and 212b, respectively, the amount of exciters to be excited accordingly increases and the amount of exciters to be transmitted to the second quantum dot 213 also increases so that the light intensity of the output light based on the discharging increases. To the contrary, when the signal lights A and B are not supplied to all the first quantum dots 212a and 212b, respectively, the amount of exciters to be excited accordingly decreases and the amount of exciters to be transmitted to the second quantum dot 213 also decreases so that the light intensity of the output light based on the discharging lowers. This indicates that the signal lights A and B to be supplied to the respective quantum dots 212a and 212b are controlled thereby to change the light intensity of the output light. That is, the light intensity of the output light assumes to be expressed as a sum of signal lights to be supplied, and if the signal lights can be extracted as output signals, an arithmetic circuit 21 can be operated as a so-called adder circuit for adding the signal lights. Further the arithmetic circuit can be applied for finding so-called OR, in which an output light is emitted if at least one of the signal lights A and B is input in the adder circuit. In this case, a threshold is set to be less than a light emission intensity based on at least one supplied signal light, and when an actual light emission intensity exceeds the threshold, the decision is “true”, and when the actual light emission intensity is less than the threshold, the decision is “false.” Thereby, as illustrated in FIG. 13A, there can be configured an arithmetic circuit for outputting so-called OR Y of the signal intensities for the inputs A and B.
There is proposed, as an arithmetic circuit using quantum dots, a circuit for NOT operation as described in JP 2006-237515 A, for example. As illustrated in FIG. 13B, the NOT operation circuit by quantum dots functions as an inverter for returning an inverse value to the input A.
Typically, the energy levels between a first quantum dot 111 and a second quantum dot 112 are different as illustrated in FIG. 14A, and thus a resonance will not occur. To the contrary, when a signal light corresponding to a gate signal is supplied to the second quantum dot 112, exciters are correspondingly excited to a gate energy level (1, 1, 1) in the second quantum dot 112. When, exciters are excited to a second energy level (1, 1, 1), a second energy level (2, 1, 1) correspondingly shifts to be lower as illustrated in FIG. 14B. Consequently, the first energy level in the first quantum dot 111 and the second energy level in the second quantum dot 112 are configured at substantially the same level. Thus, a resonance occurs between the first energy level (1, 1, 1) and the second energy level (2, 1, 1), and exciters are injected from the first quantum dot 111 having a small volume into the second quantum dot 112 having a large volume. Additionally, the exciters injected into the second quantum dot 112 transit to the energy level (1, 1, 1) in the second quantum dot 112, and are alleviated.
In this state, when an input light is supplied to the first quantum dot 111, most of the exciters excited at the first energy level (1, 1, 1) move to the second quantum dot 112 along with a movement of the resonant energy, and energy to be discharged to the lower level decreases. Consequently, a light emission intensity based on the energy discharging lowers. The light emission intensity is an output in the arithmetic circuit.
Therefore, the amount of resonant energy movement is changed to the second quantum dot 112 along with ON/OFF of a gate signal to be supplied to the second quantum dot 112, so that the state of the exciters excited at the first energy level in the first quantum dot 111 can be changed. Consequently, the amount of discharged energy to the lower level in the first quantum dot 111 can be controlled, thereby changing the light intensity of the output light. The gate signal is turned on so that the above resonant energy moves and thus the light intensity of the output light lowers, and the gate signal is turned off so that the light intensity of the output light increases. Thus, the light intensity of the output light indicates a reverse trend when the gate signal is turned on or off, and thus the arithmetic circuit can be applied as a so-called NOT operation circuit.